light problem

#1
my low beams wont turn on. i just bought the car used, then only one light was working. now none. I hear a buzz sound coming from one light nothing from the other. I have the HID Xenon headlights. Could it be a ballast or a bulb? I pulled out the bulb, but i don't know how to check if its blown. help me.
 
#2
interchane the bulbs first and see if they work, if they do then the tansformer / control unit needs replacement
 


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